This patch makes logic surrounding the test of the
transmit time stamping flag more readable.

[ RC - Rebased Takahiro's changes and wrote a commit message
  explaining the changes. ]
Signed-off-by: NTakahiro Shimizu <tshimizu818@gmail.com>
Signed-off-by: NRichard Cochran <richardcochran@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>